Duncan Smith and Osborne have been at pains to deny reports of a turf war between their departments since the chancellor first mentioned the additional savings, to come from out-of-work benefits, in a BBC interview last week.

Appearing before the Commons work and pensions committee this morning, Duncan Smith insisted that there had been no falling out with the Treasury, referring instead to "robust discussions".

But after laughing off the reports of a feud, the former Tory leader then appeared to directly undermine Osborne. Asked by committee chair Anne Begg about a variety of figures that had been "bandied about", including the £11bn savings set out in the June budget, the £4bn announced by Osborne and the £2.5bn cuts reported by the Observer on Sunday, he responded: "As with regards to figures like £4bn, I simply do not recognise that figure at all."

Osborne pitched the extra savings as an attempt to deal with those who saw claiming out-of-work benefits as a "lifestyle choice". But the chancellor failed to confirm the figure when called to the Commons to answer an emergency question on Monday.

Duncan Smith acknowledged to the committee that "every department has to play their part" but also insisted that he had not agreed to any cuts yet.

"I have not made any commitment to saving in the spending round yet because that's a matter for me, the Treasury and Downing Street," he said.

Osborne was criticised after announcing his plan to cut an additional £4bn from the welfare budget in an interview with the BBC, rather than in parliament. He also came under fire from Bob Russell, a Lib Dem backbencher, who accused him of being "unethical" for blaming "this country's ills on this small category of the population".

The relatively high short-term costs of Duncan Smith's proposals to simplify the benefits system and create incentives to work are reportedly of concern to the Treasury at a time when it is seeking to find deep cuts in public spending. Although his reforms are expected to reduce the overall welfare bill in the medium to long term they will cost billions to implement in the short term.

As well as denying knowledge of the £4bn figure, the work and pensions secretary also played down the significance of the "secret plan" to slash the welfare bill by £2.5bn for people who are disabled or too ill to work reported by the Observer. "That letter [from Osborne to Duncan Smith containing the proposals] had nothing to with our present discussions at all and should be discarded," he told the committee.

In the emergency budget in June the government announced plans to save £11bn from welfare, and at the time Osborne indicated he was looking for further savings from the welfare budget if at all possible.